In a machine tool such as a machining center, a work pallet supporting a workpiece thereon is usually positioned on a table when the workpiece is machined. In this case, the work pallet is carried to a close position above a base body fixed on the table or a base body as the table itself, and is positioned about the base body before machining the workpiece fixed on the work pallet. In the case of machining various parts for a precision machine, the machining accuracy of about 1 μm is sometimes required, and the workpiece machining accuracy is affected by errors in positioning the work pallet on the base body, and therefore, it is very important to assure the accuracy of positioning the work pallet on the base body.
For example, in a work pallet positioning and fixing device disclosed in Japanese Laid-open Publication No.2001-38564, a tapered collet is externally slidably fitted on a sleeve shaft formed at a base body, and the tapered collet is energized upward by a spring member, and at the upper end of base body formed is a receiving surface for vertically positioning the work pallet. A bush fitted on the pallet is formed with an annular tapered surface capable of externally fitting tight on the tapered collet and of abutting tight with the receiving surface.
When the work pallet is positioned and fixed on the base body, the work pallet is attracted to the base body side and fixed by means of a clamp mechanism. In this case, the tapered collet is fitted tight between the sleeve shaft and the bush, and the work pallet is horizontally positioned against the base body, and the abutment surface of the bush comes in contact with the receiving surface, then the work pallet is positioned in a vertical direction as well.
On the other hand, in a work pallet positioning and fixing device disclosed in Japanese Laid-open Publication Hei No. 8-155770, a base body is provided with a tapered cone for horizontally positioning the work pallet and a receiving surface for vertically positioning the work pallet. Also, the work pallet is provided with a tapered socket externally fitted on the tapered cone, and the tapered socket is provided with an abutment surface for contacting with the receiving surface, and the inner periphery of the tapered cone is formed with an annular groove for promoting elastic deformation.
When the work pallet is positioned on the base body, the work pallet is attracted to the base body and fixed by means of a clamp mechanism. In this case, the tapered cone is slightly elastically deformed to reduce its diameter, and the tapered socket comes in tight contact with the tapered cone, causing the work pallet to be horizontally positioned, then the abutment surface at the work pallet side comes in contact with the receiving surface of the base body, causing the work pallet to be positioned in a vertical direction as well.
However, in a positioning and fixing device disclosed in Japanese Laid-open Publication No.2001-38564, the tapered collet is configured as a part separate from the sleeve shaft and is externally slidably fitted on the sleeve shaft, which is biased upward by the spring member and stopped by a stop ring. In such a structure, the number of parts is increased and the manufacturing cost becomes expensive, and also, if chips or the like generated during machining intrude around the tapered collet, it will cause worsening of vertical movement of the tapered collet or extreme lowering of the positioning performance.
On the other hand, the positioning and fixing device disclosed in Japanese Laid-open Publication Hei No. 8-155770 is configured so that the tapered cone is elastically deformed in a direction of diametrical reduction, but the tapered cone is formed of an annular wall which is continuously annular and relatively large in thickness, and therefore, the tapered cone is hard to be elastically deformed in the direction of diametrical reduction, and it is probably unable to assure positioning accuracy when positioning in both horizontal and vertical directions. Moreover, by increasing extremely the clamping force of the clamp mechanism, it will become possible to elastically deform the tapered cone in the direction of diametrical reduction, but it results in increase of the manufacturing costs for the clamp mechanism and its relating structures.
In addition, since the tapered cone is structured to be elastically deformed in the direction of diametrical reduction and is continuously annular in shape, the tapered cone will not be locally elastically deformed when the work pallet is positioned. For example, in a case where a plurality of positioning and fixing devices are disposed on the work pallet, the axial center of the annular tapered surface of tapered socket is often slightly deflected from the axial center of the tapered cone. In such a case, when a portion in the circumferential direction of the tapered cone is locally elastically deformed, the intended positioning function will be nearly obtained, but in a case where the tapered cone is not locally elastically deformed, it will be unable to obtain the intended positioning function and also the annular tapered surface of the tapered cone is liable to wear unevenly. The same holds true in the device disclosed in the previously mentioned Japanese Laid-open Publication No.2001-38564.
The object of the present invention is to provide a work pallet positioning and fixing device capable of positioning in both horizontal and vertical directions through elastic deformation of an annular engaging portion, a work pallet positioning and fixing device having an engaging portion which is elastically deformable along the entire periphery, and a work pallet positioning and fixing device having an engaging portion which is elastically deformable even locally in a circumferential direction.